Abstract: The present invention relates to a substance of formula (I), whereby R1 is an alkyl or alkenyl group having 6 to 20 carbon atoms; R2 is H or missing, whereby O is bound via a double bond, R3 is H or an acyl group -C(O)R5, whereby R5 is an alkyl or alkylene group having 1 to 10 carbon atoms, and R4 is H or a phosphate group for use as a medicament and for use in a method of preventing or treating a subject suffering from an autoimmune disease. The present invention further relates to a method for generating regulatory T cells (Treg cells) in vitro comprising the steps of providing precursor CD4+T cells, cultivating the precursor CD4+T cells provided in step 1) in the presence of the substance as defined herein, and, optionally, isolating the generated regulatory T cells (Treg cells).